Many of you will know that Sheldon Green died suddenly, towards the end of 1995. When Ewine asked me to say a few words at this meeting, in Sheldon's memory, I thought it very appropriate to do so and did not hesitate to agree. Although several people here probably knew Sheldon personally better than I (because our careers evolved on opposite sides of the Atlantic), it is possible that no-one present is more familiar with his published work.

The rotational energy transfer of NH3, OH and D2CO in inelastic collisions with He and H2 is studied in a crossed molecular beam experiment. The molecules are prepared in a single initial state by rotational cooling in an adiabatic expansion followed by electrostatic state selection. Relative state-to-state cross sections are determined by measuring the collision induced redistribution of the population of the initial state by using state selective laser detection techniques. The results for NH3 and OH are compared to theoretical values obtained from quantum calculations. Except for NH3 — He where theory predicts a parity selection rule for transitions to the 33 and 43 states, which is not observed in the experiment, good agreement between experiment and theory is obtained.

Over the past few years, rate coefficients (k) have been measured for neutral-neutral reactions at low (down to 80 K) and ultra-low (down to 13 K) temperatures (T). The experimental methods and the results which have been obtained are reviewed. A surprisingly large number of reactions become faster at lower temperatures and have rate coefficients close to the collisional value below ca. 50 K. The factors which influence the form of k(T) are briefly discussed and proposals are made as to how rate coefficients might be estimated for use in astrochemical models.

The dynamics of the astrophysically relevant reactions OH + H2,OH+CO,N(2D)+C2H2 and O(1D)+H2 are studied using the crossed beam scattering technique. Comparisons of the experimental results with those of dynamics calculations on ab initio and semi-empirical potential energy surfaces are discussed.

The recent laboratory spectroscopy of a number of astrophysically important molecules are discussed. Examples include small molecules such as CrH, HF, FeF, SiS, CH4, H20, SiO and TiO as well as large molecules such as COO and polycyclic aromatic hydrocarbons (PAHs). Many of these examples illustrate the utility of infrared and far infrared emission spectroscopy. The close coupling of laboratory spectroscopy and molecular astronomy has led to advances in both areas.

The electronic absorption spectra of a variety of carbon chains have been identified in the laboratory. The measurements were carried out in 5 K neon matrices on mass-selected species. On the basis of the trends evident in the homologous series, it can be predicted in which region absorption bands of the carbon chains are to be expected for the not yet measured longer and isoelectronic species. The spectroscopic characteristics, photostability and structural considerations point out which and what size carbon chains could be the diffuse interstellar bands absorbers.

Current ab initio methods for determining potential energy surfaces are discussed in relation to the TiO and H2O molecules, both of which make important contributions to the opacity of M-type stars. For the TiO molecule we discuss the determination of the radiative lifetimes of the excited states and band oscillator strengths for both the triplet and singlet band systems. While the theoretical radiative lifetimes for TiO agree well with recent measurements, the band oscillator strengths differ significantly from those currently employed in opacity calculations. For the H2O molecule we discuss the current results for the ground state potential energy and dipole moment surfaces generated at NASA Ames. We show that it is necessary to account for such effects as core-valence correlation to generate a potential energy surface of near spectroscopic accuracy. The current status of our effort to establish opacity data bases for both TiO and H2O is described.

Microprobe two-step laser mass spectrometry (μL2MS) is a new mass spectrometric method in which the two essential steps of any mass spectrometric analysis, vaporization and ionization, are carried out using two independent laser sources. In the first step, the output of a pulsed infrared laser is focused on the sample to cause rapid heating in the spot area illuminated, which is typically 40 μm by 40 μm. In the second step, the output of a pulsed ultraviolet laser causes (1+1) resonance-enhanced multiphoton ionization (REMPI) of those desorbed neutral molecules that (1) are able to absorb this UV wavelength and (2) whose ionization potential is less than the energy of two photons of this UV wavelength. The resulting ions are then mass analyzed in a reflectron time-of-flight apparatus. Under suitable conditions fragmentation can be avoided in both the vaporization and ionization steps so that μL2MS can be applied to the analysis of a mixture of molecules. Applications of μL2MS to meteorite samples are presented as a means of detecting trace amounts of certain organic molecules present in complex materials without prior sample preparation, extraction, purification, and separation steps. Moreover, this analysis can be carried out with micrometer spatial resolution so that in favorable cases the presence or absence of certain molecules can be correlated to mineralogical features of the sample.

Computational modeling is discussed of some interstellar surface processes. The surface of interstellar grains is envisaged as covered by at least several layers of weakly bonded molecular material. Simulated amorphous ice particles were used to model interaction of such surfaces with gas, including sticking of H and D atoms, and adsorption of H2. A possibility of microporosity of interstellar ice mantles is discussed.

We review the techniques which are applied to study by means of laboratory simulation the chemical and physical processes taking place in ices in interstellar dense clouds. We summarize the current situation with regard to spectroscopy of interstellar ice analogs and with regard to the simulation of chemical processes such as modification of the ices by UV photons and surface reactions between reactive atoms and molecules. It is discussed how such data can be used to explore the potential of the icy mantles both as a record of the various chemical conditions that occur in dense clouds and as an environment for unique interstellar chemical processes.

During the last years our knowledge about the structure and chemical composition of cosmic dust grains has very much deepened mainly driven by the rapid progress in infrared observations, laboratory experiments, and a much better theoretical treatment of grain evolution and the microphysical interactions of solid particles with gas, radiation, and with each other. This contribution will review the progress in the understanding of cosmic grains especially concentrating on the different grain populations, the physical processes which they link together, and the morphological and chemical structure of the particles.

Polycyclic Aromatic Hydrocarbons (PAHs) are undoubtedly present in large amounts in many different astronomical objects as well as in the diffuse interstellar medium. They are detected through their infrared vibrational transitions in the mid-infared but are expected to be observable in other regions of the spectrum such as the UV, the visible and the far-infrared. PAHs are suggested to be the most abundant free organic molecules in the interstellar environment. In the laboratory, investigations have been undertaken to provide the tools for the interpretation of astronomical observations. For example, the recent detection of the overtone of the well-known 3.28 μm band around 1.67 μm has led to the determination of a relatively large minimum average size of about 60 atoms. In the far-infrared, above 15 up to 100 μm, expected transitions from laboratory measurements are scattered all over this region, making the detection of these lines difficult. In the diffuse medium, a complex aromatic network is expected on interstellar grains and its similarity with carbon extracts from primitive meteorites is suggestive of a link between interstellar matter and primitive Solar System bodies, in particular comets.

We report on the discovery of new solid state emission features in the ISO-SWS spectra of evolved oxygen-rich stars. These features appear in cool dust shells and are probably due to crystalline forms of silicates, and crystalline water ice.

The Infrared Space Observatory (ISO) is opening the 2.5 to 200 μm band for detailed infrared spectroscopy. Concentrating on ionic and molecular emission lines observed with the Short Wavelength Spectrometer (SWS) we discuss ISO's first results on Galactic and extragalactic sources.

We have detected four far-infrared emission lines of water vapor toward the evolved star W Hydrae, using the Short Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO). This is the first detection of thermal water vapor emission from a circumstellar outflow.

We present ISO Long Wavelength Spectrometer (LWS) grating spectra of AGB and post-AGB stars. The LWS instrument and the calibration procedures are described by Clegg et al. (1996) and Swinyard et al. (1996).

The first detection of thermal H2O emission from an Herbig-Haro flow was made with the LWS (Long Wavelength Spectrometer) aboard ISO (Infrared Space Observatory). In addition to H2O, rotational lines of CO and OH as well as lines from [O I] and [C II] were also recorded from HH 54. These observations are consistent with the concept of interstellar shock waves, but a quantitative unifying shock model, capable of explaining all observations, has yet to be developed.

The large amount of water vapour in the Earth's atmosphere makes it extremely difficult to observe water in interstellar space, except under unusual conditions (e.g., masers). The Short Wavelength Spectrometer (SWS; de Graauw et al. 1996) on board the Infrared Space Observatory (ISO) provides an unique opportunity to study the ro-vibrational fundamental bands of water in absorption against bright infrared continuum sources. Such absorption line data have several advantages over emission observations. For example, only a pencil beam to the infrared source is probed, which minimizes the complications due to the small-scale physical and chemical heterogeneity of star-forming regions. In addition, lines from all rotational levels are present within a vibrational band, so that the complete spectrum yields a direct estimate of the level populations and excitation temperature along the line of sight. Other molecules, such as CO, HCN and C2H2, can be observed with the same technique, leading to accurate relative abundances. The main limitation of the ISO-SWS grating is its low spectral resolving power, λ/Δ λ ≈ 1350 at 6 μm, since the lines are intrinsically much narrower. This limits the sensitivity, so that only high column densities can be detected, and complicates the interpretation because detectable lines are often saturated. The low spectral resolution also blends the lines, making it difficult to determine the equivalent widths unambiguously. The ISO-SWS is therefore particularly sensitive to molecules in warm gas with relatively large line widths (Δ V > 3 km s−1).

CO2 is one of the simplest carbon- and oxygen-containing molecules, for which little is known in interstellar space, because of its high abundance in the Earth's atmosphere. Only indirect searches of the chemically related HOCO+ ion have been possible from the ground (Thaddeus et al. 1981; Minh et al. 1988, 1991). The ISO Short Wavelength Spectrometer (SWS) (de Graauw et al. 1996a) opens up the possibility to search for the infraredactive asymmetric stretch and bending modes of gas-phase CO2 around 4.3 and 15.0 μm toward bright infrared sources.

Absorption-line studies of diffuse interstellar clouds have recently been invigorated at ultraviolet wavelengths with the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope and at optical wavelengths with efficient high resolution spectrographs and CCD detectors. These instruments have made it possible to explore new regimes in determining the atomic and molecular abundances of diffuse clouds and tracing the velocity structure of interstellar lines. In the case of the atomic gas, the abundances of elements as diverse as oxygen and krypton have been accurately measured with GHRS and are consistent with a local ISM whose metallicity is about 2/3 that of the Sun. GHRS has also provided new insight on molecular processes in diffuse clouds through observations of CO, C2, HCl, and vibrationally-excited H2. With velocity resolutions of 0.3 km s−1 now attainable, optical spectra of species like CH, CH+, and CN are beginning to probe the physical characteristics of diffuse clouds in detail. Similar spectra of interstellar Na I toward resolvable binary star systems have recently revealed a rich variety of small-scale structure in the cold diffuse gas.